This invention relates to the field of displaying imagery on a display screen. In particular, the present invention relates to a multi-color display screen with optically programmable memory capable of greatly enhancing quality of the display image and relaxing the information bandwidth required of the display controlling equipment.
A conventional projection display system functions as follows: imaging light emitted from a light source is projected on a projection screen, and viewers observe the imagery projected on the screen.
Most present display systems completely update each frame of the image at a rate of 30-60 Hz. This practice is historical, based on the properties of typical cathode ray tube displays. The full-screen refresh requirement makes the updating and driving technology associated with ultra-high resolution displays very challenging. For example, the high I/O bandwidths will require driver electronics having very high clock rates, which will result in high power dissipation. In typical motion video applications, however, the information at every pixel does not change so rapidly. Thus, if the display can remember the information content of each pixel, only those pixels having changed content need to be updated. Furthermore, those updates typically can occur at a much slower rate than 60 Hz. Incidentally, those applications, such as synthetic aperture radar (SAR) imaging, that generate data that require the highest display resolution also generally have the slowest update rates.
Advanced imaging sensors generate large amount of data that will require ultra-high resolution displays if their entire images are to be viewed. For example, staring infrared sensors with 25 million pixel resolution are being developed. Present day IMAX movie formats provide 30 million pixels per frame. Also, data generated by synthetic aperture radar (SAR) sensors can have over 1 billion pixels. Thus, the display system must be capable of handling many more pixels if such sensor data are to be displayed.
The incorporation of memory into a display screen means that only the portions of an image that change from one frame to the next needs to be updated. Some formats for video data compression (such as MPEG-2) are based on representing only the changes that occur between frames of an image and are compatible with such an approach for the display. The MPEG-2 format can result in a reduction in data rate of more than 50:1. Other compression formats such as MPEG-4 project even greater reductions in data rate.
The incorporation of memory into the display screen makes possible display systems that have a very large number of pixels for ultra-high resolution. The resolution of the HDTV format (with more than 2 million pixels) matches the visual acuity of a person having 20/20 vision if the size of the display is small compared to its distance from the viewer. However, if a person is viewing a large area screen, such as in typical home-entertainment settings, improvements in the resolution of the screen may be perceptible. In that case, a display system capable of more pixels often is preferred. Even more pixels would be needed if the viewer is located closer to the screen, for a more immersive experience. For example, the screen would need to have 21.2 million pixels to match the visual acuity of that person if she is located two meters away from the screen. See. D. G. Hopper, “1000× difference between current displays and capability of human visual system: payoff potential for affordable defense systems,” SPIE Proceedings, vol. 4022 (2000), pp. 378-389.
As an example, consider a projector that produces a beam that is scanned across the many pixel locations of a screen. This projector might contain some means (such as rotating mirrors) to raster scan the output beam across the surface of the screen. A typical HDTV screen has 1920×1080 pixels. At a frame rate of 60 per second with progressive (instead of interlaced) scanning of the vertical lines for the image, such a video format would require each pixel to be modulated at a rate of approximately 125 MHz if every pixel must be updated in each frame.
Lasers or LEDs could be used as the light sources and many types of those devices can be modulated at a rate of 125 MHz or higher. The HDTV format has acceptable image resolution if the viewer is suitably distant from the display screen. However many more pixels would be needed for applications that desire even larger display areas with the same visual resolution (i.e., permitting shorter viewing distances). A large area display capable of displaying entire SAR images may require 20,000×10,000 pixels or more. In this case, the lasers or LEDs would need to be modulated at a rate of 12.5 GHz or higher. Such a modulation rate is currently not available for most laser wavelengths and is not possible with LEDs. Of course the large area display could be divided into sections of smaller area, with a separate projector used to illuminate each section. However, the use of a large number of projectors by such a tiled approach may not be desirable or may be too costly in some applications.
The modulation rate requirement on the lasers/LEDs can be relaxed substantially if the screen has memory and those optical sources are used only to reprogram or change the image information stored in selected pixels. For the example given above of a ultra-high resolution large area display that has 20,000×10,000 pixels, the kind of data encoding provided by the MPEG-2 format means the modulation rate for the lasers or LEDs that program the screen could be much lower, even as low as approximately 250 MHz. This is because the programming projector need only update those pixels of the display screen whose video content has changed from one frame to the next. Having the image memory located in the screen rather than in the projector also means that the video memory in the projector could be much smaller. This reduction can be achieved if the MPEG-2 input is used to directly control the programming projector rather than to update a video memory that contains the information for the entire screen area.
Information relevant to attempts to address these problems can be found in U.S. Pat. Nos. 5,618,654; 5,691,091; 5,988,822 and 6,366,388. Furthermore, see: B. Gnade, A. Akinwande, R. Shashidhar and J. Larimer, “Display bandwidth reduction via latched pixels and processing at the pixel,” SPIE Proceedings, vol. 4712 (2002), pp. 313-317; T. Tsujioka, M. Kume and M. Irie, “Superlow-power readout characteristics of photochromic memory,” Japanese Journal of Applied Physics, vol. 34 (1995) pp. 6439-6443; E. Molinari, et al., “Photochromic polymers for erasable focal plane masks and re-writable volume phase holographic gratings,” SPIE Proceedings, vol. 4485 (2002), pp. 469-477; N. Hampp, M. Sanio and K. Anderle, “High-resolution direct-view displays based on the biological photochromic material bacteriorhodopsin,” SPIE Proceedings, vol. 3636 (1999), pp. 40-47; A. J. Myles, T. J. Wigglesworth and N. R. Branda, “A multi-addressable photochromic 1,2-dithienylcyclopentene-phenoxynaphthacene-quinone hybrid,” Advanced Materials, vol. 15, no. 9 (2003), pp. 745-748; N. Davies, M. McCormick and L. Yang, “Three-dimensional imaging systems: a new development,” Applied Optics, vol. 27, n. 21 (1988) pp. 4520-4528; P. Harman, “Retroreflective screens and their applications to auto-stereoscopic displays,” SPIE Proceedings, vol. 3012 (1997) pp. 145-153. However, each one of these references suffers from one or more of the following disadvantages: electrically active (requires power to maintain memory); single-color display only; requirement of multi-peak spectral sensitive photochromic materials; incapable of displaying multiple images on retro-reflective screen;
For the foregoing reasons, there is a need for an optically programmable screen with memory that provides wavelengths specific modulation of the reflection or transmission of the display screen, enhanced resolution and reduced information bandwidth. There is also a need for a multi-color display screen with memory to render a full color display. Furthermore, there is a need for a display screen with memory that is electrically passive and can select between different wavelengths of programming light or different wavelength components of illumination light.